Blowing a gas into a granular filter bed

ABSTRACT

A gas is blown into a filter (1) having a granular filter bed such that the gas flows into the bed along at least one elongate free edge disposed in the bed.

The present invention relates to a method and apparatus for blowing agas into a granular filter bed of a type where the filter medium movesdownwards against the flow of a suspension that is to be treated in thefilter bed. The invention is particularly suitable for blowing air intoa filter bed intended for the nitrification of wastewater.

In recent times there has been an increased need of nitrogen reductionin wastewater, not in the least due to governmental edicts. In Sweden, anitrogen reduction of at least 50% will be demanded in 1992 for certainsensitive parts of its coastline, and the prerequisites for a 75%reduction are to be investigated. Nitrogen reduction is usuallybiologic, and in two steps. In the first step, ammonium compounds areoxidized to nitrates, i.e. nitrification, and in the second stepnitrates are reduced to nitrogen gas, i.e. denitrification. Theprocesses are usually carried out under agitation in sludge basins.

Greater interest has been shown in recent times for biological reductionusing granular filter beds, for nitrification as well asdenitrification. Great advantages may be obtained by biologicalpurification in filter beds, as compared with the basin processes, therebeing, inter alia, no need for a number of sedimentation basins forseparating and recirculating sludge. However, there are difficulties inthe use of filter beds for nitrogen reduction, and one such difficultywill be dealt with below.

The first nitrogen reduction step, i.e. nitrification, takes placeaccording to the general formula:

    NH.sup.+.sub.4 +1.86 O.sub.2 +0.1 CO.sub.2 =0.021 C.sub.5 H.sub.7 O.sub.2 N+0.941 H.sub.2 O+0.98 NO.sup.-.sub.3 +1.98 H.sup.+.

The ammonium compounds are reduced by oxygen to nitrates. The term 0.021C₅ H₇ O₂ N corresponds to the production of new bacteria. The filter bedused may be a continuously operating sand filter, where the sand at thebottom of the bed is removed for washing and returned to the top of thebed. The wastewater that is to be treated is supplied at the bottom ofthe bed, and the treated water, i.e. the clear solution, is taken offfrom the top, or above the bed. Air is used as a source of oxygen, andis supplied at the bottom region of the bed via a number of horizontallydisposed pipes having a large number of holes with a diameter of lessthan 1 mm. A sand filter operating continuously in this way isdescribed, e.g. in the French patent specification No. 2 623 794.

The bacteria attach themselves to the grains of sand. If washing thesand is not performed too intensively, a bacterius strain remains afterwashing sufficiently for the nitrification. The strain grows in size asthe sand in the filter bed moves downwards. Bacteria, which in such acase can form long threads, also attach themselves to the airinletpipes, causing the small air holes to become blocked. This causesthe air supply to be cut off locally, or to decrease in magnitude.Another common cause of blocking up these holes is the deposition oflime. The difficulties caused by blockage of the air supply holes havebeen found impossible to overcome, except by dismantling the pipes afterthey have been in service for some time in order to clean them, which isexpensive and troublesome.

The present invention relates to a new method and new apparatus forsupplying gas into a granular filter bed of the kind mentioned in theintroduction. In accordance with the invention, the gas is conveyed outinto the filter bed by being continuously distributed along at least oneelongated edge, flows round this edge and upwards within the filter bed,whereby the inflow being such that a substantially uniform distributionof the gas through the filter bed is created.

With the inflow of the gas in accordance with the invention, the abilityof the bacteria to block the gas supply is simply and effectivelyinhibited. In addition, it is essential for obtaining effectiveutilisation of the invention that substantially uniform distribution ofthe gas blown into the bed is obtained, e.g. for nitrification there isdesired as uniform as possible oxygenation of the bed.

It is to be emphasised that the invention is only applicable to acontinuously operating filter bed, where the filtering medium adjacentto, and immediately below the inflow edge is continuously renewed and isnot afforded the opportunity of filling up the space behind the edge.

In a preferred embodiment of the invention, the mentioned elongate edgeis formed as a hood, open downwards and otherwise closed, excepting theplace for gas supply to it. In this embodiment the hood has a shapesimilar to a ridged roof with end walls.

In order to achieve the above-mentioned uniform distribution of theincoming gas, a preferred embodiment utilises a constructively simpleimplementation with the mentioned elongate edge disposed substantiallyhorizontal in the filter bed, thus ensuring uniform distribution of thegas flowing into the filter bed along the edge. The invention may befurther improved by providing means for regulating the setting of theedge during operation.

It is also important that the gas is supplied to the hood at a low rateof flow, otherwise local over-pressure can occur in it, which wouldcause uneven gas flow to the filter bed. Flow barriers, e.g. bafflesthat smooth out the gas flows under the hood, can be arranged forfurther reducing the risk of local over-pressure. It has been found thatthe dynamic pressure of the gas at all points under the hood should bebelow 5 mm water column to obtain uniform gas flow to the filter bed.The dynamic air pressure under the hood should preferably be less than 2mm water column.

An embodiment of the invention will now be described in more detail, andwith reference to the accompanying drawing, which schematicallyillustrates a filter apparatus incorporating the described embodiment.

In the drawing there is illustrated a continuously operating filter bed1, which is intended to be used for nitrification. The bed is assumed tobe activated by autotrophic bacteria. A pipe 2, for conveying pollutedfilter medium, in this case sand, passes through the central portions ofthe bed, from its lowest level to a washing apparatus 3. Air is used fortransporting the sand to the washing apparatus 3, whereby the air istaken down centrally in a pipe 4 disposed concentrically around the pipe2 and provided with an inlet 5. The washed sand is returned to the topof the filter bed, and moves downwards in the bed as the sand at thebottom is taken away for washing. The wastewater that is to be treatedin the bed is taken via a pipe 6 down into the bottom portion of thebed, e.g. under a plurality of horizontal arms 7 departing from the pipe6. A cone 8 guides the downward movement of the sand for achieving itsuniform cross-sectional distribution. The treated liquid, containingnitrates dissolved in the water, is taken out (not shown) somewhereabove-the bed. The wastewater is oxygenated during this counterflowprocess between water and sand. Oxygenation suitably takes place usingair that is introduced into the filter bed under a plurality of elongatehoods 9 disposed horizontally in the bed just above the wastewaterinlet. The hoods have a shape similar to that of a ridged roof, roundthe bottom edge of which the air flows into the bed. Each of the hoodsis provided at one end with a pipe 10, through which air is supplied. Itshall be possible to regulate the amount of air to each hood, which iswhy each one has its own air supply pipe, and is also provided with aregulating valve (not shown). In addition, there is preferably somemeans, operable exterior to the bed, for setting the inflow edge of thehoods as horizontal as possible, e.g. a rod with a threaded connectionto its respective hood.

The amount of air required for acceptable nitrification in a sand bedwould appear to be double as much volumetrically as the amount ofwastewater. A surface load of the bed of 10 m/h wastewater requiresabout 20 m/h air loading. The bubbles ascend substantially verticallythrough the sand bed after they have left the hood edge. The number andsize of the hoods should be dimensioned so that high air flow rates areavoided, since these might cause fluidisation inside the bed.

The stream of air bubbles ascending straight up from a hood edge pushesto one side some of the water in the bed. When the stream ceases, theempty space thus formed is filled by the water that flows back again,and this lateral movement of the water may be amplified by continuallyturning the air flow on and off, thus creating better oxygenationconditions for the water.

A preferred disposition of hoods has been described above. However, aplurality of different ways of disposing the hoods is possible forobtaining the inflow edge essential to the invention. For example, thehoods may extend in a circular configuration in the bed, and besubdivided into a plurality of sections, each having its own air supply.The hood edges may also be toothed instead of being smooth, as describedabove.

The filter has been described up to now as continuously operating. Thisdoes not mean that each operational phase is continuous. For example,the upward conveyance of polluted filtering medium (in this case sand)can be arranged to be stopped while building up a bacterium strain inthe bed.

It should also be noted that although blowing air into a filter bedintended for the nitrification of wastewater has been speciallydescribed here, there are other fields of use as well, where blowing gasinto a granular filter bed could come into question, and where thepresent invention could be used. With respect to the hoods, these do notneed to be formed with a straight outflow edge, as illustrated on thedrawing, but may have some other shape, e.g. circular for a hood in theform of a cone.

I claim:
 1. A method of blowing gas into a granular filter bed of acontinuously operating filtering apparatus which comprises providing ata bottom end of the filter bed an enclosure having an open bottom and anelongate bottom edge and directing the gas outwardly from said enclosureover the length of said edge and upwardly through the bed.
 2. A methodas claimed in claim 1 wherein said enclosure comprises a downwardly openelongate hood with a closed top and said edge comprises a lowerhorizontal edge of the hood, further wherein said gas is introduced intothe hood for flow over said edge and upwardly through the bed.
 3. Amethod as claimed in claim 1 wherein the gas is blown intermittentlyinto the filter bed.
 4. A method as claimed in claim 1 wherein the gashas a dynamic pressure of less than 5 mm water column prior to flowthrough the filter bed.
 5. A method as claimed in claim 4 wherein saiddynamic pressure is 2 mm water column
 6. Apparatus for blowing a gasinto a granular filter bed of a continuously operating filter apparatuscomprising an elongate hood arranged in a bottom end of the filter bed,the hood being closed upwardly and open downwardly, such as to form anelongate lower inflow edge for the gas, said edge being disposedsubstantially horizontally in the filter bed during operation of thefilter apparatus and means for introducing gas into said hood for flowover said edge and upwardly through the filter bed.
 7. Apparatus asclaimed in claim 6, wherein said inflow edge is straight.
 8. Apparatusas claimed in claim 6, wherein the hood is V-shaped in cross section. 9.Apparatus as claimed in claim 6, wherein the inflow edge is circular.10. Apparatus as claimed in claim 1, including means for setting theposition of the hood edge.
 11. Apparatus as claimed in claim 1,including a valve in a gas supply line, for supplying the gasintermittently to the hood.
 12. Apparatus as claimed in claim 1,including flow obstructing means for dampening the gas flows under thehood.